474 



The Tunicates, or Ascidians 



of them form from their own secretions a transparent gelatinous 

 envelope called a " house. " This has two apertures and a long 

 chamber "in which the tail has room to vibrate." 



The order consists of a single small family, Appendiculariidcz. 

 The lowest type is known as Kowalevskia, a minute creature 

 without heart or intestine found floating in the Mediterranean. 

 It is in many respects the simplest in structure among Chordate 

 animals. Oikopleura (Fig. 288) is another genus of this group. 



Ascidiacea. In the Ascidiacea the adult is usually attached 

 to some object, and the two apertures are placed near each 

 other by the obliteration of the caudal area. The form has 

 been compared to a "leathern bottle with two spouts." 



The suborder Ascidice simplices includes the solitary Ascid- 

 ians or "sea-squirts," common on our shores, as well as the 

 social forms in which an individual is sur- 

 rounded by its buds. The common name 

 arises from the fact that when touched they 

 contract, squirting water from both aper- 

 tures. The Ascidiidce comprise the most 

 familiar solitary forms, some of them the 

 largest of the Tunicates and represented on 

 most coasts. In the Molgulidce and most 

 Ascidics composites the young hatch out in 

 the cloaca, from which "these tadpoles 

 swim out as yellow atoms," while in a new 

 genus, Eukerdmania, described by Ritter, 

 fromt he coast of California, the embryos are 

 retained through their whole larval stage in 



FIG. 280. Ascidia ad- .. ... ,. , ^, * 



hcerens Ritter. Glacier the oviduct of the parent. They form, ac- 



^ After cording to Kingsley, adhesive processes on 

 the body, but those of Molgula cannot use 

 them in becoming attached to rocks, since they are entirely in- 

 closed in a peculiar envelope. This envelope is after a while very 

 adhesive, and if the little tadpole happens to touch any part 

 of himself to a stone or shell he is fastened for life. Thus " I 

 have frequently seen them adhere by the tail, while the anterior 

 part was making the most violent struggles to escape. Soon, 

 however, they settle down contentedly, absorb the tail, and 

 in a few weeks assume the adult structure." 



